In a digitally controlled printing system, a print medium is directed through a series of components. The print medium can be cut sheet or a continuous web. As the print medium moves through the printing system, a printing material such as a liquid (e.g., ink) or dry powder (e.g., toner), is applied to the print medium by one or more print stations to produce a multi-color output on paper or plastic. In other applications, the print stations in a printing system can be used to produce an electrical circuit (e.g., flex-circuits or touch panels), by applying conductive or insulating materials to a substrate.
In commercial inkjet printing systems, the print stations include multiple printheads that jet ink onto the print medium as the print medium is physically moved through the printing system at a high rate of speed (e.g., 1000 feet/minute). A reservoir containing ink or some other material is usually behind each nozzle plate in a print station. The ink streams through the nozzles in the nozzle plates when the reservoirs are pressurized. The jettable liquid is applied to the print medium as it passes by the print station by applying a control signal to each nozzle to jet a small amount of liquid (drops) such as an ink onto the print medium to form a single picture element commonly referred to as a “pixel.” Print stations typically contain nozzle arrays at 600 nozzles/inch capable of printing 600 dots/inch. The nozzle arrays are arranged within each print station across the full width of the print medium. Repeated jetting of ink (controlled by the control signals applied to each nozzle) will produce individual picture elements (pixels) in the direction of the print medium movement. The timing for the nozzle control signals is typically linked to the speed of the print medium such that a constant distance (e.g., 1/600 inch) between successive pixels on print medium is achieved even while the printing process is changing speed.
In the print stations of commercial electrophotographic printing systems, a photoconductive drum or belt is first electrostatically charged. When the photoconductor is selectively exposed to light, typically from a light modulated scanned laser beam or an array of LEDs, the exposed regions become conductive, causing the charge to be depleted from those regions of the photoconductor. In the development stage, charged toner particles are attracted to the charged portions of the photoconductor (i.e., the portions that have not been exposed to light). The toner image is then transferred to the print medium, either directly or via an intermediate transfer substrate. The toner is then fused to the print medium using a combination of heat and pressure.
The printheads in each print station in commercial printing systems typically print only one color. A print station is provided for each colored ink to print color image content. For example, many printing system include four print stations for printing cyan, magenta, yellow and black inks. The print content is formed by printing the colored inks sequentially, one image plane at a time, each image plane using only one type of colorant. In the example of a four color print, all four image planes together on a single page form the print content. The content printed by an individual print station is sometimes referred to as an image plane.
Similarly, the production of a three-layer electrical circuit, such as a touch panel, requires a print system with three print station (e.g., a conducting material, followed by an insulating material, followed by another conductive material). The print content of a three-layer conductive circuit is formed on the substrate by printing the conducting, insulating and conducting inks sequentially, one image plane at a time. In the example of a three-layer circuit, all three image planes together on a single page form the printed content.
For multi-color image content or the proper function of the printed circuit, the image planes on each page need to be aligned, or registered with each other so that the overlapping ink colors produce a quality multi-color image or a quality multi-layer electrical circuit.
Color registration errors can be partitioned into different types. Examples of color registration errors include, but are not limited to, an image plane (i.e., color plane) having a linear translation with respect to another image plane, an image plane being rotated with respect to another image plane, an image plane skewed with respect to another image plane forming a trapezoidal shape and an image plane being stretched, contracted, or both stretched and contracted in different regions or in different directions with respect to another image plane.
There are several variables that contribute to the registration errors in image plane alignment including physical properties of the print medium, conveyance of print medium, ink application system, ink coverage, and drying of ink. Color registration errors are typically managed by controlling these variables. However, controlling these variables can often restrict the range of desired print applications. For example, image plane to image plane registration errors will typically become larger than desired as paper weight for the print application is reduced, when ink coverage is increased, or when the amount of ink coverage becomes more variable between printed documents. These limitations compromise the range of suitable applications for inkjet printing systems. Therefore, there exists a need to characterize registration errors during the printing process in real-time and develop the means to correct the individual pixel placement for each image plane (real-time), so that registration errors between image planes are minimized during the printing process.